Flight Management Computers (FMCs) are used to allow the entry, by a flight crew, of a pre-planned route from the origin of the flight to the destination. This lateral route is established by entering a series of waypoints, usually identified by aircraft navigation stations such as VHF Omnidirectional Radio (VOR) stations, Automatic Direction Finder (ADF) stations, Instrument Landing System (ILS) stations providing localizer and glide slope information as well as marker beacons to annunciate, for example, distance to a runway, or the like.
Each of the series of waypoints from the origin of the flight to the destination is entered into the FMC in sequence, and the FMC may display the lateral route, or a portion of it, and may also, through cooperation with an automatic pilot, actually fly the aircraft along the entered lateral route, overflying each waypoint in succession until the terminal point of the flight is reached. This route is flown by the FMC using a lateral control law and sequencing logic. The FMC creates errors to the path and maneuvers the aircraft to zero out those errors.
Often, however, the flight crew may deviate from the pre-planned route, either on their own or as a result of an Air Traffic Control (ATC) instruction to so deviate due to nearby traffic or other routing considerations. In such event, the crew tactically maneuvers the aircraft to a new heading either by engaging the autopilot heading select mode and dialing in a new heading, or by flying the aircraft manually, thus causing the aircraft to fly off the pre-planned route. Once the air traffic conflict or other cause for deviation from the planned route is resolved, ATC allows the aircraft to return to the pre-planned FMC route.
Most FMCs include a Control and Display Unit (CDU). The CDU is the unit that actually stores the flight plan entered by the crew. Some CDUs have a feature known as a “Legs Page” that displays the information relating to each of the legs of the flight plan including waypoints, headings, navigation station identifiers, etc. The then-active leg is at the top of the page and the CDU determines when the wayline (which is a line perpendicular to the terminus of the leg) for that leg is passed, at which point the next successive leg becomes the active leg and appears at the top of the legs page. Even if the FMC is in Heading mode, the CDU keeps track of the active leg so long as the aircraft is within some number of nautical miles (for example, twenty one nautical miles) of the flight plan path. If the aircraft is more than twenty nautical miles from the planned flight path, the CDU no longer keeps track of the active leg. The twenty one nautical miles is, of course, somewhat arbitrary and could be some other distance in accordance with system design preferences.
Typically, in order to return to the pre-planned FMC route a series of activities on the part of the flight crew must take place. The crew must:                1. Decide on the most appropriate waypoint to return to in the pre-planned route. The most appropriate waypoint may be the then-active waypoint, the waypoint closest to the aircraft, the waypoint closest to, yet ahead of, the aircraft's present position, a down-path waypoint that results in the lowest recapture bank angle, or an intercept course to the leg into any of the aforementioned waypoints.        2. Down select this waypoint into the Scratch-Pad of a Control and Display Unit (CDU).        3. Up-select this waypoint from the Scratch Pad to the top of the route on the Legs Page of the CDU (or alternatively, enter an intercept course to the active leg).        4. Verify the modification of the lateral map display.        5. Press EXEC on the CDU.        6. Select LNAV (Lateral Navigation Mode) on the Mode Control Panel (MCP) so that the FMC commands the autopilot to recapture the pre-planned route.        
In addition to the actions noted above, the crew must also create an efficient path back to the pre-planned route by taking the shortest path back to the pre-planned route using as few banking maneuvers as possible to assure passenger comfort. In addition to the Direct-To path described, the crew can also elect to fly an intercept course into the active waypoint. Each of these activities requires “head down” time by the crew, which is a safety issue, and requires substantial time to make the necessary calculations to return the aircraft to its original pre-planned route as computed by the FMC.
If the aircraft flies past one or more waypoints, the FMC commonly does not sequence those waypoints if certain conditions exist, for example, if the aircraft is more than twenty one nautical miles from the pre-planned flight path. This is an additional problem faced by the crew since they must realize this condition and perform additional keystrokes on the CDU to remove the non-sequenced legs from the pre-planned route. It is also quite common that the crew will be unaware when clearance back to the pre-planned route will be granted. Therefore, once clearance is granted by ATC, the crew is suddenly faced with performing the above actions, which further diminishes their alertness to safety to handle a heads down action.
Accordingly, it is desirable to provide an automated method of selecting a new active waypoint and providing a return to path maneuver that can eliminate a number of the steps noted above. In addition, it is desirable to provide a method for reducing errors associated with returning to the pre-planned route and minimize the heads down time spent resolving the route. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.